The present invention relates to a projection lens system for use in a projection television.
In a projection television, the pictures produced by projection cathode-ray tubes (CRTs) for three colors, red, green and blue, are projected by means of three associated lens systems to form an enlarged color television picture on a screen. In order to design a thinner or smaller projection television, it is necessary to use projection lenses that feature a wide view angle and a large aperture ratio and that provide good imaging performance.
Many versions of projection lenses have heretofore been proposed to meet these requirements and they include a system that solely uses glass lenses in order to insure good quality by achieving high precision of machining; a system that solely uses plastic lenses in order to reduce the production cost while achieving a greater aperture ratio; and a hybrid system that uses glass lenses in combination with aspheric plastic lenses.
The first mentioned projection lens system that is solely composed of glass lenses have the disadvantage that its size and the production cost will increase if one attempts to improve the imaging performance without compromising its large aperture ratio. Further, it suffers the problem of substantial deterioration in marginal performance. The second system which is solely composed of aspheric plastic lenses is suitable for the purpose of providing a large aperture ratio but the precision of machining that can be attained with plastic lenses is not as high as in the case of glass lenses, so it is difficult to fabricate a system that is capable of operating to closely meet the designed performance. Further, plastic lens are sensitive to temperature variations and any changes in their refractive index or shape will result in a change in the position of focal point, leading to deterioration in their imaging performance.
It is generally held that the individual lenses used in a projection television to separately project the pictures produced in red, green and blue CRTs need not be subjected to acromatism. In spectral measurements of the projection CRTs in actual use, however, a broad emission spectrum is observed for blue light whereas spurious emission spectra are observed for green and red light. Therefore, non-achromatic projection lenses are poor in imaging performance for respective colors and are incapable of insuring satisfactory color reproduction.